Publications

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2012
Chen, X, Shearer PM, Abercrombie RE.  2012.  Spatial migration of earthquakes within seismic clusters in Southern California: Evidence for fluid diffusion. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb008973   AbstractWebsite

Seismicity within many earthquake swarms is observed to migrate slowly with time, which may reflect event triggering due to slow fault slip or fluid flow. We search for this behavior in Southern California by applying a weighted least squares method to quantify event migration within 69 previously observed seismicity bursts. We obtain best-fitting migration directions and velocities, and compute a statistical migration significance s(m) for each burst using a bootstrap resampling method. We define 37 bursts with s(m) >= 0.8 as the migration group, and 32 bursts with s(m) < 0.8 as the non-migration group. To explore differences between the two groups, for each burst we compute effective stress drop (Delta sigma(quasi), the ratio between total moment and radius), the skew of the moment release time series (mu), the timing of the largest event (t(max)), and the distance separation between the first half and second half of the sequence (d(s)). As expected, the migration group features larger d(s) and lower Delta sigma(quasi), consistent with higher migration significance. It also features lower mu and higher t(max), similar to observations from swarms in the Salton Trough, while the non-migration group is more similar to main shock-aftershock sequences. To explore possible fluid involvement, we model the migration behavior with the fluid diffusion equation, and identify 18 bursts with diffusion coefficients ranging from 0.01 to 0.8 m(2)/s, with the majority below 0.16 m(2)/s. The obtained diffusion coefficients and migration behavior are similar to the Reservoir-induced seismicity beneath the Acu reservoir in Brazil. The majority of normal faulting events are associated with these 18 bursts, while the non-migration group has the most reverse faulting events, indicating a possible link between sequence type and focal mechanism.

2006
Vidale, JE, Shearer PM.  2006.  A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004034   AbstractWebsite

[ 1] We investigate the cause of seismicity bursts by examining a waveform-relocated catalog for southern California between 1984 and 2002 and systematically identifying 71 isolated sequences of 40 or more earthquakes occurring within a 2-km-radius volume and a 4-week interval. Fifty-seven of the 71 bursts are difficult to interpret as primarily a main shock and its Omori-law-abiding foreshocks and aftershocks because they exhibit a more complicated evolution in space, time, and magnitude; we identify 18 of these sequences as particularly swarm-like. Evidence against a simple cascade of elastic stress triggering includes the presence of an interval of steady seismicity rate, the tendency of the largest event to strike later in the sequence, the large spatial extent of some of the swarms compared to their cumulative moment, and the weak correlation between the number of events in each burst and the magnitude of the largest event in each burst. Shallow sequences and normal faulting mechanism sequences are most likely to be swarm-like. The tendencies of the hypocenters in the swarm-like sequences to occur on vertical planes and expand over time suggest pore fluid pressure fluctuations as the most likely mechanism driving the swarm-like seismicity bursts. However, episodic aseismic slip could also be at least partly responsible and might provide a more compelling explanation for the steady rate of seismicity during swarms, whereas fluid pressure perturbations might be expected to diminish more rapidly with time. Both aftershock-like and swarm-like seismicity bursts are distributed across the entire study region, indicating that they are a general feature of tectonic faulting, rather than limited to a few geological conditions such as volcanic or geothermal areas.

Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.